" There is a thing called the blood feud. All societies have them, even the West Saxons have them, despite their vaunted piety. Kill a member of my family and I shall kill one of yours, and so it goes on, generation after generation or until one family is all dead, and Kjartan had just wished a blood feud on himself. I did not know how, I did not know where, I could not know when, but I would revenge Ragnar. I swore it that night. "

― Bernard Cornwell , The Last Kingdom (The Saxon Stories, #1)